caused. Tens of millions of birds died of influenza and hundreds of millions were culled to protect humans.

The chapter begins with a reconstruction of the descent of the virus that infected and killed humans in Thailand and Vietnam during the winter of 2003–2004 from the H5N1 virus first known to have infected humans (in Hong Kong in 1997). These findings indicate that domestic ducks in southern China played a central role in the generation and maintenance of H5N1 and that wild birds spread the virus across Asia, to the point where it is now endemic in the region—an ecological niche from which it now presents a long-term pandemic threat to humans.

The chapter continues with descriptions of the approach taken by two countries most severely affected by the H5N1 epidemic: Thailand and Vietnam. Each country’s circumstances and their handling of the epidemic—beyond the use of common, time-tested strategies for detecting and stamping-out of infection—were unique, as illustrated in these contributions. The diversity of these responses, and their resulting outcomes, offer important lessons for the control of future avian flu outbreaks—a key protection against a human pandemic.

Correspondence and requests for materials should be addressed to Y. Guan (yguan@hkucc.hku.hk). The sequences reported in this paper have been deposited in GenBank under accession numbers AY651320–AY651758.

2

We acknowledge K. Stöhr and the World Health Organization for facilitating the study; L.J. Zhang, C.L. Cheung, and Y.H.C. Leung for technical assistance; N. Ng and colleagues for provision of computing facilities; and T.M. Ellis, K. Dyrting, W. Wong, P. Li, and C. Li of the Department of Agriculture, Fisheries and Conservation of Hong Kong for their support of field work, and W. Lim, for virus isolates. We also thank S. Naron for editorial assistance. These studies were supported by a grant from the National Institutes of Health, a grant from The Wellcome Trust, the Ellison Foundation, the Li Ka Shing Foundation, and grants from the Research Grants Council of Hong Kong.